When you think about it, a combustible lemon is possible to create. However, one of the problems I see is this: Lemons have plant cells. Plant cells have water. Water doesn't burn, so if you were to alter the chemical composition of the lemon and remove the water....COMBUSTIBLE LEMONS FOR ALL!

"When life gives you lemons, don't make lemonade. Make life take the lemons back! Get mad! I don't want your damn lemons! What am I supposed to do with these? Demand to see life's manager! Make life rue the day it thought it could give Cave Johnson lemons! Do you know who I am? I'm the man who's gonna burn your house down! With the lemons! I'm gonna get my engineers to invent a combustible lemon that burns your house down!"―Cave Johnson

"I've just been informed that I should not have told you about the control group. The boys back in the lab said I should stop making pre-recorded messages. That gave me an idea: keep making pre-recorded messages. I pay the bills around here, I can talk about the control group all damn day."